FinFET with a semiconductor strip as a base

ABSTRACT

A method includes forming a first hard mask over a semiconductor substrate, etching the semiconductor substrate to form recesses, with a semiconductor strip located between two neighboring ones of the recesses, forming a second hard mask on sidewalls of the semiconductor strip, performing a first anisotropic etch on the second hard mask to remove horizontal portions of the second hard mask, and performing a second anisotropic etch on the semiconductor substrate using the first hard mask and vertical portions of the second hard mask as an etching mask to extend the recesses down. The method further includes removing the vertical portions of the second hard mask, and forming isolation regions in the recesses. The isolation regions are recessed, and a portion of the semiconductor strip between the isolation regions protrudes higher than the isolation regions to form a semiconductor fin.

PRIORITY CLAIM AND CROSS-REFERENCE

This application claims the benefit of the following provisionally filedU.S. Patent application: Application Ser. No. 62/312,763, filed Mar. 24,2016, and entitled “Novel Fin Pattern to Achieve High Height and SmallPitch without Fin Collapse and Peeling;” which application is herebyincorporated herein by reference.

BACKGROUND

Technological advances in Integrated Circuit (IC) materials and designhave produced generations of ICs where each generation has smaller andmore complex circuits than the previous generations. In the course of ICevolution, functional density (for example, the number of interconnecteddevices per chip area) has generally increased while geometry sizes havedecreased. This scaling down process generally provides benefits byincreasing production efficiency and lowering associated costs.

Such scaling down has also increased the complexity of processing andmanufacturing ICs and, for these advances to be realized, similardevelopments in IC processing and manufacturing are needed. For example,Fin Field-Effect Transistors (FinFETs) have been introduced to replaceplanar transistors. The structures of FinFETs and methods of fabricatingFinFETs are being developed.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIGS. 1 through 12 are perspective views and cross-sectional views ofintermediate stages in the formation of a Fin Field-Effect Transistor(FinFET) in accordance with some embodiments.

FIGS. 13 through 16 illustrate the cross-sectional views of FinFETs inaccordance with some embodiments.

FIG. 17 illustrates a process flow for forming a FinFET in accordancewith some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact. In addition,the present disclosure may repeat reference numerals and/or letters inthe various examples. This repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

Further, spatially relative terms, such as “underlying,” “below,”“lower,” “overlying,” “upper” and the like, may be used herein for easeof description to describe one element or feature's relationship toanother element(s) or feature(s) as illustrated in the figures. Thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. The apparatus may be otherwiseoriented (rotated 90 degrees or at other orientations) and the spatiallyrelative descriptors used herein may likewise be interpretedaccordingly.

Fin Field-Effect Transistors (FinFETs) and the methods of forming thesame are provided in accordance with various exemplary embodiments. Theintermediate stages of forming the FinFETs are illustrated. Thevariations of the embodiments are discussed. Throughout the variousviews and illustrative embodiments, like reference numbers are used todesignate like elements.

FIGS. 1 through 12 illustrate the perspective views and cross-sectionalviews of intermediate stages in the formation of a FinFET in accordancewith some embodiments. The steps shown in FIG. 1 through 12 are alsoillustrated schematically in the process flow 400 shown in FIG. 17. Inthe subsequent discussion, the process steps shown in FIGS. 1 through 12are discussed referring to the process steps in FIG. 17.

FIG. 1 illustrates a perspective view of substrate 20, which is a partof a wafer. Substrate 20 is a semiconductor substrate such as a siliconsubstrate, a silicon carbon substrate, a silicon-on-insulator substrate,or a substrate formed of other semiconductor materials. Substrate 20 mayalso include other semiconductor materials such as III-V compoundsemiconductor materials. Substrate 20 may be lightly doped with a p-typeor an n-type impurity.

Pad oxide 22 and hard mask 24 are formed over semiconductor substrate20. In accordance with some embodiments of the present disclosure, padoxide 22 is formed of silicon oxide, which may be formed by oxidizing asurface layer of semiconductor substrate 20. Hard mask 24 may be formedof silicon nitride, silicon oxynitride, silicon carbide, siliconcarbo-nitride, or the like.

Next, as shown in FIG. 2, hard mask 24, pad oxide 22, and substrate 20are patterned to form trenches 26. Accordingly, semiconductor strips 128and 228 are formed. Trenches 26 extend into semiconductor substrate 20,and have lengthwise directions parallel to each other. In accordancewith some embodiments of the present disclosure, depth D1 of trenches 26are in the range between about 80 nm and about 130 nm. It is appreciatedthat the values recited throughout the description are examples, anddifferent values may also be adopted without changing the principle ofthe present disclosure.

Substrate 20 includes portions in two device regions, namely multi-findevice region 100 and single-fin device region 200. Semiconductor strips128 and 228 are formed in regions 100 and 200, respectively. Inmulti-fin device region 100, a FinFET formed on a plurality ofsemiconductor fins is to be formed. In single-fin device region 200, aFinFET including only a single semiconductor fin is to be formed.Throughout the description, two fins are illustrated in multi-fin deviceregion 100 as an example, and more fins may be formed in multi-findevice region 100.

Referring to FIG. 3A, hard mask layers 130 and 230 are formed inmulti-fin device region 100 and single-fin device region 200,respectively. Hard mask layers 130 and 230 are formed simultaneously ina same deposition process, and are formed on the top surfaces andsidewalls of semiconductor strips 128 and 228. Furthermore, hard masklayers 130 and 230 extend on the top surface of semiconductor substratein trenches 26. The deposition method is selected so that the resultinghard mask layers 130 and 230 are substantially conformal, with thethickness T1 of the vertical portions equal to or substantially equal tothickness T2 of the horizontal portions. For example, thickness T1 maybe between about 80 percent and 100 percent of thickness T2. Inaccordance with some embodiments of the present disclosure, thedeposition method includes Atomic Layer Deposition (ALD), Low PressureChemical Vapor Deposition (LPCVD), or the like.

Hard mask layers 130 and 230 may be formed of aluminum oxide (A1 ₂O₃),silicon nitride, silicon oxide, or the like. Hard mask layer 130includes two vertical portions between semiconductor strips 128, each onthe sidewall of one of semiconductor strips 128. The two neighboringvertical portions of hard mask layer 130, although appear to be joinedwith a seam therebetween, are actually not joined. FIG. 3B illustratescross-sectional views of the structures in multi-fin device region 100and single-fin device region 200, which shows gap 129 between twoneighboring vertical portions of hard mask layer 130. Gap 129 has a veryhigh aspect ratio, which may be greater than about 15, and may bebetween about 15 and about 30. It is appreciated that gap 129 is theunfilled portion of the respective trench 26. In subsequent discussion,the term “outer trenches” are used to refer to trenches 26 that are onthe outer sides (the illustrated left side and right side) of theoutmost semiconductor strips 128.

Next, referring to FIG. 4, a first anisotropic etch is performed toremove the horizontal portions of hard mask layers 130 and 230. Thefirst anisotropic etch is performed through dry etch using, for example,hydrogen fluoride (HF) as an etching gas. The vertical portions of hardmask layers 130 and 230 on the sidewalls of semiconductor strips 128 and228 remain after the first anisotropic etch.

As a result of the etching, the top surfaces of hard masks 24 areexposed. Furthermore, the top surfaces of semiconductor substrate 20 atthe bottom of trenches 26 are also exposed. After the first anisotropicetch, a second anisotropic etch is performed to further etchsemiconductor substrate 20, so that trenches 26 further extend lowerthan the bottom edges of hard mask layers 130 and 230. In accordancewith some embodiments of the present disclosure, depth D2 of trenches 26is increased to be in the range between about 120 nm and about 160 nm.The depth difference (D2-D1) may be in the range between about 30 nm andabout 50 nm.

In accordance with some embodiments of the present disclosure, thesecond anisotropic etch is performed using an etchant gas different fromthe etchant gas used in the first anisotropic etch. In accordance withalternative embodiments, the first and the second anisotropic etches areperformed using a same etchant gas such as a fluorine-containing gas ora chlorine-containing gas. The first and the second anisotropic etchsteps may be performed in a same process chamber with no break inbetween. Throughout the description, the portions of semiconductorsubstrate over the bottoms of the extended outer trenches 26 andunderlying semiconductor strips 128 and 228, are referred to assemiconductor strip bases 132 and 232, respectively, which are the basesover which semiconductor strips 128 and 228 are resided. Semiconductorstrip bases 132 and 232 are over the bulk portions of semiconductorsubstrate 20.

In the second etching step, hard masks 24 and the vertical portions ofhard mask layer 130 and 230 are in combination used as the etching maskfor the second anisotropic etch, and hence the sidewalls ofsemiconductor strip bases 132 and 232 may be vertically aligned to theouter sidewalls of the vertical portions of hard mask layers 130 and230. Depending on the etching process, there may be some undercutsformed, resulting in the exposed sidewalls of semiconductor strip bases132 and 232 to be tilted and recessed from the respective outer edges ofthe vertical portions of hard mask layers 130 and 230.

Referring back to FIG. 3B, in gap 129, horizontal portion 130′ of hardmask layer 130 is at the bottom of, and is exposed to, gap 129.Horizontal portion 130′ may have a thickness equal to thickness T2,which is the thickness of portion 130″ in outer trenches 26. Outertrenches 26 have a lower aspect ratio than gap 129. In accordance withsome embodiments, due to the high aspect ratio of gap 129 (FIG. 3B), inthe first and the second anisotropic etch processes, the etching rate ofthe bottom portion 130′ of hard mask layer 130 under gap 129 is muchlower than the etching rate of bottom portions 130″ and 230″ in outertrenches 26.

Depending on the aspect ratios of gap 129 and trenches 26, the thicknessof hard mask layer 130, and the process conditions of the etching,various results may occur, as shown in FIGS. 13 through 16. Also, theresults shown in FIGS. 13 through 16 may co-exist on the samesemiconductor substrate. For example, it is possible that in the firstand the second anisotropic etch processes, portion 130′ (FIG. 3B) is notetched-through. As a result, the portion of semiconductor substrate 20directly underling portion 130′ is not etched. Portion 130′ may also beetched-through at a time delayed from the time portions 130″ areetched-through. Accordingly, the portion of semiconductor substrate 20directly underling portion 130′ starts to be etched when the portions ofsubstrate 20 directly underling portions 130″ have been etched for aperiod of time. Accordingly, the portion of semiconductor substrate 20directly underlying portion 130′ is etched for a shorter period of timethan the portion of semiconductor substrate 20 directly underlyingportions 130″. As a result, the trench 26 between two neighboringsemiconductor strips 128 may also extend down into semiconductorsubstrate 20 during the second etch, however, with a smaller depth thanouter trenches 26.

Next, a wet etch is performed to remove remaining portions of hard masklayers 130 and 230, thus exposing the sidewalls of semiconductor stripbases 132 and 232. The resulting structure is shown in FIG. 5. Inmulti-fin device region 100, a plurality of semiconductor strips 128stands on the same semiconductor strip base 132, while a single strip228 stands on semiconductor strip base 232. Throughout the description,semiconductor strip bases 132 and 232 may be considered as parts ofsubstrate 20, or may be considered as parts over substrate 20.

Next, as shown in FIG. 6, isolation regions 133 and 233, which may beShallow Trench Isolation (STI) regions, are formed in trenches 26 (FIG.5). The formation may include filling trenches 26 with a dielectricmaterial(s), for example, silicon oxide using Flowable Chemical VaporDeposition (FCVD), and performing a CMP to level the top surface of thedielectric material with the top surface of hard masks 24. After theCMP, hard masks 24 (FIG. 5) is removed. Alternatively, the polish stopson the top surfaces of semiconductor strips 128 and 228. In a top viewof the structure shown in FIG. 6, each of semiconductor strip bases 132and 232 may be a strip encircled by the respective STI regions 133 and233, or may be a strip with the opposite ends connected to semiconductorsubstrate 20.

Next, referring to FIG. 7, STI regions 133 and 233 are recessed, so thatthe top surfaces of the resulting STI regions 133 and 233 are lower thanthe top surfaces of semiconductor strips 128 and 228. Throughout thedescription, the portions of semiconductor strips 128 and 228 higherthan the top surfaces of STI regions 133 and 233 are referred to assemiconductor fins 134 and 234, respectively. The top surfaces of theremaining STI regions 133 and 233 are further higher than the topsurfaces of semiconductor strip bases 132 and 232. Throughout thedescription, the portions of STI regions 133 between two neighboringsemiconductor strips 128 are referred to as inner STI regions 133, whilethe STI regions 133 on the outer sides of the outmost semiconductorstrips 128 are referred to as outer STI regions. STI regions 233 areouter STI regions.

Referring to FIG. 8, dummy gate stacks 146 and 246 are formed onsemiconductor fins 134 and 234, respectively. Dummy gate stacks 146 and246 cover the middle portions of semiconductor fins 134 and 234,respectively, leaving the opposite end portions not covered. Inaccordance with some embodiments of the present disclosure, dummy gatestacks 146 and 246 include dummy gate electrodes 48, which may be formedof, for example, polysilicon. A dummy oxide layer (not shown) may (ormay not) be formed before the formation of dummy gate electrodes 48.Hard masks 50 are formed over dummy gate electrodes 48, and are used asetching masks for forming dummy gate electrodes 48. Hard masks 50 mayinclude silicon nitride and/or silicon oxide, for example, and may be asingle layer or a composite layer including a plurality of layers. Forexample, hard masks 50 may include pad oxides 50A and silicon nitridelayers 50B over pad oxides 50A. Pad oxides 50A may be formed of siliconoxide, which may be formed by oxidizing the top surface layers of dummygate electrodes 48. The formation of dummy gate stacks 146 and 246 mayinclude depositing the respective layers as blanket layers, and thenetching the blanket layers. Dummy gate stacks 146 and 246 may havelengthwise directions substantially perpendicular to the lengthwisedirection of the respective semiconductor fins 134 and 234.

Referring further to FIG. 8, spacer layer 54 is formed in both deviceregions 100 and 200. In accordance with some embodiments of the presentdisclosure, spacer layer 54 is formed of silicon oxide, silicon nitride,silicon oxynitride, or multi-layers thereof. For example, spacer layer54 may include a silicon oxide layer, and a silicon nitride layer overthe silicon oxide layer. Spacer layer 54 is formed using a conformaldeposition method such as ALD.

FIG. 9 illustrates the etching of spacer layer 54 to form gate spacers156 and 256, which are on the sidewalls of dummy gate stacks 146 and246, respectively. In accordance with some embodiments, an anisotropicetch is performed to etch spacer layer 54. The horizontal portions ofspacer layer 54 are removed. In addition, since the heights ofsemiconductor fins 134 and 234 are lower than that of dummy gate stacks146 and 246, the heights of the vertical portions of spacer layer 54 onthe sidewalls of semiconductor fins 134 and 234 are relatively small,and hence may be removed. On the other hand, the vertical portions ofspacer layer 54 on the sidewalls of dummy gate stacks 146 and 246 arerelatively small, and have portions remaining after the etching. Theremaining portions of spacer layer 54 are gate spacers 156 and 256. Dueto the etching, the top surfaces of gate spacers 156 and 256 arerecessed from the top surfaces of dummy gate stacks 146 and 246.

In the formation of gate spacers 156 and 256, the top surfaces of theexposed STI regions 133 and 233 may also be recessed by the etchant andthe chemicals used in the respective etching and cleaning processes. Toensure that semiconductor strip bases 132 and 232 are not exposed afterthe recessing of STI regions 133 and 233, the portions of STI regions133 and 233 over top surfaces of semiconductor strip bases 132 and 232are designed to have an adequate thickness when the structure shown inFIG. 7 is formed.

After the formation of gate spacers 156 and 256, as also shown in FIG.9, a source/drain epitaxy is performed to grow epitaxy semiconductorregions 158 and 258 on the exposed end portions of semiconductor fins134 and 234, respectively. Epitaxy regions 158 and 258 and the endportions of semiconductor fins 134 and 234 in combination form thesource/drain regions of the respective FinFETs. In accordance with someembodiments in which the resulting FinFET is an n-type FinFET, epitaxyregions 158 and 258 comprise silicon phosphorous (SiP) orphosphorous-doped silicon carbon (SiCP). In accordance with alternativeembodiments in which the resulting FinFET is a p-type FinFET, epitaxyregions 158 and 258 comprise silicon germanium (SiGe), and a p-typeimpurity such as boron or indium is in-situ doped during the epitaxy. Animplantation may also be performed to dope an n-type or p-type impurityinto the source/drain regions of the respective n-type FinFETs or p-typeFinFETs.

FIG. 10 illustrates the formation of source/drain silicide regions 159and 259 on epitaxy regions 158 and 258, respectively. The formation ofsource/drain silicide regions 159 and 259 includes forming a blanketmetal layer (not shown) on the structure shown in FIG. 9, wherein themetal layer is formed on the top surfaces and sidewalls of epitaxyregions 158 and 258. An anneal is performed to react the metal layerwith epitaxy regions 158 and 258 to form silicide regions 159 and 259.The unreacted metal is then removed, leaving silicide regions 159 and259.

Next, as shown in FIG. 11, etch stop layer 160 and 260 and Inter-LayerDielectric (ILD) 62 are formed. A CMP is then performed to level the topsurfaces of ILD 62, dummy gate stacks 146 and 246 (FIG. 10), and gatespacers 156 and 256 (FIG. 10) with each other. In accordance with someembodiments, the CMP is performed until dummy gate electrodes 48, whichare formed of polysilicon, are exposed. In accordance with alternativeembodiments, the CMP stops on hard masks 50 (FIG. 1), which are formedof polysilicon, are exposed.

Next, dummy gate stacks 146 and 246 as shown in FIG. 10 are removed inan etching step, so that recesses (not shown, occupied by replacementgates 164 and 264 as shown in FIG. 12) are formed to extend into ILD 62,as shown in FIG. 12. The portions of Semiconductor fins 134 and 234covered by dummy gate stacks 146 and 246 are exposed to the recesses.

Next, replacement gate stacks 166 and 266 are formed in the recessesleft by the removed dummy gate electrodes, as also shown in FIG. 12.Replacement gate stacks 166 and 266 may include a plurality ofdielectric layers as replacement gate dielectrics 163 and 263 (FIGS. 13through 16), and a plurality of conductive layers as replacement gateelectrodes 164 and 264 (FIGS. 13 through 16). In accordance with someembodiments, the formation of each of replacement gate dielectrics 162and 262 includes performing an interfacial (dielectric) layer, and thenforming a high-k dielectric layer on the interfacial layer. Theinterfacial layer may include silicon oxide formed by treating theexposed surface of semiconductor fins 134 and 234 in a chemicalsolution, so that semiconductor fins 134 and 234 are oxidized to form achemical oxide (silicon oxide). The high-k dielectric is then depositedon the interfacial layer. In accordance with some embodiments, thehigh-k dielectric has a k value greater than about 7.0, and may includea metal oxide or a silicate of Hf, Al, Zr, La, and the like.

Gate electrodes 164 and 264 (FIGS. 13 through 16) in replacement gatestacks 166 and 266 are formed over the replacement gate dielectrics 163and 263, respectively. Replacement gate electrodes 164 and 264 mayinclude a metal-containing material such as TiN, TaN, TaC, Co, Ru, Al,Cu, W, combinations thereof, or multi-layers thereof. After theformation of the replacement gate dielectrics and the replacement gateelectrodes, a planarization such as a CMP is performed to remove excessportions over ILD 62. The result structure includes multi-fin FinFET 170in multi-fin device region 100, and single-fin FinFET 270 in single-findevice region 200.

FIGS. 13 through 16 illustrate the cross-sectional views of someportions of FinFETs 170 and 270 in FIG. 15, wherein the cross-sectionalview are obtained from vertical planes perpendicular to thechannel-length directions of FinFETs 170 and 270. In each of FIGS. 13through 16, the inner surfaces and bottom surfaces of outer STI regions133 (the STI region 133 on the left of the leftmost semiconductor fin134 and the STI region 133 on the right of the rightmost semiconductorfin 134) are symmetric to each other. This is because the profiles ofouter STI regions 133 are generated by the leftmost vertical portion andthe rightmost vertical portion of hard mask layers 130 (FIG. 4), whichare symmetric to each other. The inner sidewalls and bottom surfaces ofthe left STI region 233 are also symmetric to that of the right STIregion 233, which symmetry is the result of the symmetry of the verticalportions of hard mask 230 (FIG. 4) on opposite sidewalls ofsemiconductor strip 228.

FIG. 13 illustrates the cross-sectional views of some portions ofFinFETs 170 and 270 in accordance with some embodiments. Semiconductorstrip bases 132 and 232 have substantially vertical sidewalls 132A and232A, which are connected to the substantially planar top surfaces 132Band 232B, respectively. The sidewalls of semiconductor strips 128 and228 form steps with top surfaces 132B and sidewalls 132A ofsemiconductor strip base 132. Inner STI region 133 has its bottomsurface in contact with top surface 132B of semiconductor strip base132. Each of STI regions 233 and outer STI regions 133 may have twosubstantially planar bottom surfaces at different levels, with thehigher one being in contact with the planar top surfaces 132B and 232Bof semiconductor strip bases 132 and 232, respectively, and the lowerones being in contact with the planar top surfaces of semiconductorsubstrate 20.

FIG. 14 illustrates the cross-sectional views of some portions ofFinFETs 170 and 270 in accordance with some embodiments. Semiconductorstrip bases 132 and 232 have titled sidewalls 132A and 232A, whereintilt angles α may be smaller than about 80 degrees. Titled sidewalls132A and 232A are connected to the substantially planar top surfaces132B and 232B. Titled sidewalls 132A and 232A may also be substantiallystraight. The sidewalls of semiconductor strips 128 also form steps withplanar top surfaces 132B and tilted sidewalls 132A of semiconductorstrip base 132. The sidewalls of semiconductor strip 228 also form stepswith planar top surfaces 232B and tilted sidewalls 232A of semiconductorstrip base 232. Inner STI region 133 also has a planar bottom surface incontact with the top surface of semiconductor strip base 132.

FIG. 15 illustrates the cross-sectional views of some portions ofFinFETs 170 and 270 in accordance with some embodiments. Theseembodiments are similar to the embodiments shown in FIG. 14, except thatthe portion of semiconductor substrate 20 directly under inner STIregion 133 is etched. Accordingly, inner STI region 133 extends intosemiconductor strip base 132. The bottom surfaces of inner STI region133 may have a V-shape in the cross-sectional view. The bottom tip ofthe V-shape is higher than the bottommost surface of outer STI regions133.

FIG. 16 illustrates the cross-sectional views of some portions ofFinFETs 170 and 270 in accordance with some embodiments. Theseembodiments are similar to the embodiments shown in FIG. 15, except thatthe portion of semiconductor substrate 20 directly under inner STIregion 133 is etched deeper, and the bottom tip of inner STI region 133is coplanar with the bottom planar surface of outer STI regions 133.Accordingly, semiconductor strip base 132 is effectively separated intoa plurality of discrete semiconductor strip bases, each having a singlesemiconductor strip 128 over it. The bottom surface of inner STI region133 may have a V-shape or a U-shape in the cross-sectional view. TheU-shape has opposite sidewalls being straight and tilted, and the bottomsurface being planar. The opposite sidewalls of the discrete portions ofsemiconductor strip base 132 may have an asymmetric profile due to thedifferent etching of portions 130′ and 130″ in FIG. 4.

A plurality of dimensions is provided for the embodiments in FIGS. 13through 16 in accordance with some exemplary embodiments. It isappreciated that these values are examples, and may be changed todifferent values. For example, in FIGS. 13 through 16, distance “a” isequal to distance “c1” and “c2” (which are further greater than about 1nm) since the sidewall portions of hard mask layers 130 and 230 havesubstantially the same width. Distance “b” between neighboringsemiconductor strips 128 is greater than two times the distance “a” toallow a gap therein.

The following relationships of the dimensions shown in FIGS. 14 through16 may also be adopted:1 nm<a<=b/2d=a+0˜8 nma=c1=c2,d=d1=d2d1=c1=0˜8 nmd2=c2+0˜8 nm

In addition, some exemplary values of the dimensions are provided inTables 1 and 2, wherein the values in Table 1 are for the structureshown in FIGS. 13 and 14, and the values in Table 2 are for thestructure shown in FIGS. 15 and 16. It is appreciated that the providedvalues are merely examples.

TABLE 1 Dimension Values FH  10 nm~100 nm SH 120 nm~160 nm ISH and ISH′ 80 nm~130 nm FH/SH 0.3~0.6 ISH/SH 0.5~0.8

TABLE 2 Dimension Values FH  10 nm~100 nm SH 120 nm~160 nm ISH′  80nm~130 nm ISH 100 nm~160 nm FH/SH 0.3~0.6 ISH/SH 0.7~1.0

The embodiments of the present disclosure have some advantageousfeatures. By forming semiconductor strip bases underlying semiconductorstrips/fins, the heights of semiconductor strips may be maintained whilethe aspect ratio of the semiconductor strips is reduced. Accordingly,the problems that will otherwise occur to high-aspect ratiosemiconductor strips are eliminated. For example, in the cleaningprocesses of conventional processes, the high-aspect ratio semiconductorstrips may bend or lean over and stick to each other, resulting in yieldloss or degradation in the performance of the FinFETs. In accordancewith the embodiments of the present disclosure, by reducing the aspectratios of semiconductor strips, semiconductor strips are less likely tobend or lean, and the problems are at least reduced, and possiblyeliminated.

In accordance with some embodiments of the present disclosure, a methodincludes forming a first hard mask over a semiconductor substrate,etching the semiconductor substrate to form recesses, with asemiconductor strip located between two neighboring ones of therecesses, forming a second hard mask on sidewalls of the semiconductorstrip, performing a first anisotropic etch on the second hard mask toremove horizontal portions of the second hard mask, and performing asecond anisotropic etch on the semiconductor substrate using the firsthard mask and vertical portions of the second hard mask as an etchingmask to extend the recesses below a bottom end of the vertical portionsof the second hard mask. The method further includes removing thevertical portions of the second hard mask, and forming isolation regionsin the recesses. The isolation regions are recessed, and a portion ofthe semiconductor strip between the isolation regions protrudes higherthan the isolation regions to form a semiconductor fin. A gate stack isformed on a top surface and sidewalls of the semiconductor fin to form aFinFET.

In accordance with some embodiments of the present disclosure, a methodincludes forming a semiconductor strip by etching a semiconductorsubstrate to form recesses on opposite sides of the semiconductor strip,forming a hard mask layer on sidewalls of the semiconductor strip,performing a first etch on the hard mask layer to remove horizontalportions of the hard mask layer, with vertical portions of the hard masklayer remaining, performing a second etch on the semiconductor substrateusing the vertical portions of the hard mask layer as parts of anetching mask to extend the recesses below a bottom end of the verticalportions of the hard mask layer, removing the vertical portions of thehard mask layer, forming isolation regions in the recesses, andrecessing the isolation regions. A portion of the semiconductor stripbetween the isolation regions protrudes higher than the isolationregions to form a semiconductor fin. A gate stack is formed on a topsurface and sidewalls of the semiconductor fin to form a FinFET, whichis a single-Fin FinFET.

In accordance with some embodiments of the present disclosure, a deviceincludes a semiconductor substrate, which includes a bulk portion, and asemiconductor strip base over and joined to the bulk portion. Thesemiconductor strip base is elongated. The device further includes asemiconductor strip over and joined to the semiconductor strip base,with the semiconductor strip being narrower than the semiconductor stripbase. A semiconductor fin is over and joined to the semiconductor strip.Isolation regions extend into the semiconductor substrate. The isolationregions include a first bottom surface contacting a first top surface ofthe semiconductor strip base, and a second bottom surface contacting atop surface of the bulk portion of the semiconductor substrate. Thesecond bottom surface is lower than the first bottom surface.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A method comprising: forming a first hard maskover a semiconductor substrate; etching the semiconductor substrate toform recesses, with a plurality of semiconductor strips located betweentwo neighboring ones of the recesses; forming a second hard mask onsidewalls of the semiconductor strip; performing a first anisotropicetch on the second hard mask to remove first horizontal portions of thesecond hard mask; performing a second anisotropic etch on thesemiconductor substrate using the first hard mask and vertical portionsof the second hard mask remaining after the first anisotropic etch as anetching mask to extend the recesses below a bottom end of the verticalportions of the second hard mask, wherein a horizontal portion of thesecond hard mask between two neighboring ones of the plurality ofsemiconductor strips is not etched-through when the second anisotropicetch is started; removing the vertical portions of the second hard mask;forming isolation regions in the recesses; recessing the isolationregions, wherein a portion of the semiconductor strip between theisolation regions protrudes higher than the isolation regions to form asemiconductor fin; and forming a gate stack on a top surface andsidewalls of the semiconductor fin to form a Fin Field-Effect Transistor(FinFET).
 2. The method of claim 1, wherein the FinFET is a single-finFinFET.
 3. The method of claim 1, wherein the FinFET is a multi-finFinFET, and the gate stack covers a plurality of semiconductor fins,with the gate stack formed on sidewalls of the plurality ofsemiconductor fins.
 4. The method of claim 1, wherein the horizontalportion of the second hard mask between neighboring ones of theplurality of semiconductor strips is not etched-through when the secondanisotropic etch is finished.
 5. The method of claim 1, wherein theisolation regions comprise: first planar bottom surfaces level with abottom surface of one of the plurality of semiconductor strips; andsecond planar bottom surfaces lower than the first planar bottomsurfaces; and straight edges connecting the first planar bottom surfacesto respective ones of the second planar bottom surfaces.
 6. The methodof claim 1, wherein after the isolation regions are recessed, topsurfaces of remaining portions of the isolation regions are higher thana bottom of one of the plurality of semiconductor strips.
 7. The methodof claim 1, wherein the first anisotropic etch and the secondanisotropic etch are performed using a same etchant gas.
 8. The methodof claim 1, wherein the first anisotropic etch and the secondanisotropic etch are performed using different etchant gases.
 9. Themethod of claim 1, wherein during the first anisotropic etch and thesecond anisotropic etch, the horizontal portion of the second hard maskis exposed to respective etching gases.
 10. A method comprising: forminga semiconductor strip by etching a semiconductor substrate to formrecesses, wherein the recesses are on opposite sides of thesemiconductor strip, wherein portions of the semiconductor substratehave a first top surface and a second top surface, each directlyunderlying one of the recesses: forming a hard mask layer on sidewallsof the semiconductor strip; performing a first etch on the hard masklayer to remove some horizontal portions of the hard mask layer, withvertical portions of the hard mask layer remaining; performing a secondetch on the semiconductor substrate using the vertical portions of thehard mask layer as parts of an etching mask to extend the recesses belowa bottom end of the vertical portions of the hard mask layer, whereinthe first top surface of the semiconductor substrate is recessed and thesecond top surface of the semiconductor substrate is protected by ahorizontal portion of the hard mask layer in the second etch; removingthe vertical portions of the hard mask layer; forming isolation regionsin the recesses; recessing the isolation regions, wherein a portion ofthe semiconductor strip between the isolation regions protrudes higherthan the isolation regions to form a semiconductor fin; and forming agate stack on a top surface and sidewalls of the semiconductor fin toform a Fin Field-Effect Transistor (FinFET), wherein the FinFET is asingle-Fin FinFET.
 11. The method of claim 10, wherein the first etchand the second etch are performed using a same etchant gas.
 12. Themethod of claim 10, wherein the first etch and the second etch areperformed using different etchant gases.
 13. The method of claim 10,wherein the first etch and the second etch are performed usinganisotropic etch.
 14. The method of claim 10, wherein after theisolation regions are recessed and the gate stack is formed, topsurfaces of remaining portions of the isolation regions are higher thana bottom of the semiconductor strip.
 15. The method of claim 10, whereinduring the second etch, the horizontal portion of the hard mask layer isexposed to a respective etching gas.
 16. A method comprising: forming ahard mask layer on sidewalls and top surfaces of a plurality ofsemiconductor strips, wherein the plurality of semiconductor stripsprotrudes higher than a bulk portion of a semiconductor substrate;performing a first etching to remove some horizontal portions of thehard mask layer, wherein after the first etching, a horizontal portionof the hard mask layer between two of the plurality of semiconductorstrips remains; performing a second etching to etch the semiconductorsubstrate and to form a semiconductor base using remaining portions ofthe hard mask layer as an etching mask; removing remaining portions ofthe hard mask layer; depositing isolation regions between the pluralityof semiconductor strips, wherein the isolation regions further compriseportions contacting outer sidewalls of the semiconductor base; recessingthe isolation regions, wherein top portions of the plurality ofsemiconductor strips protrude higher than the isolation regions to formsemiconductor fins; and forming a gate stack on top surfaces andsidewalls of the semiconductor fins to form a Fin Field-EffectTransistor (FinFET).
 17. The method of claim 16, wherein during thefirst etching and the second etching, a continuous region including theplurality of semiconductor strips and regions between the plurality ofsemiconductor strips is not covered by any etching mask in addition tothe hard mask layer.
 18. The method of claim 16, wherein differentetchants are used to etch the hard mask layer and the bulk portion ofthe semiconductor substrate.
 19. The method of claim 16, wherein afterthe isolation regions are recessed and the gate stack is formed, topsurfaces of remaining portions of the isolation regions are higher thanbottom surfaces of the plurality of semiconductor strips.
 20. The methodof claim 16, wherein during the first etching and the second etching,the horizontal portion of the hard mask layer is exposed to respectiveetching gases.